This invention pertains generally to the control of welding processes and particularly to a method for controlling gas tungsten arc welding by measuring and controlling weld penetration.
Arc welding, and particularly gas tungsten arc (GTA) welding, is perhaps one of the most widely used manufacturing processes in the world. As a consequence, great efforts have been made to automate the process and, in particular, to use robotic welders in order to increase productivity and to improve the uniformity and quality of these welds. However, continued development of automated welding processes, particularly those carried out by general purpose manufacturing robots, require continuous information relevant to the quality of the weld being made. This information must, of necessity, be furnished on a real time basis, as the weld is being made, in order to be used as part of a feed-back mechanism to control critical parameters of the welding process.
A significant variable that is extremely useful in determining the quality of a welded joint is the penetration depth of the weld. As is known in the art, there is a relationship between the characteristics of the molten weld pool and the penetration achieved by the welding process. Therefore, in order to produce uniform, high quality welds, generally, and GTA welds, in particular, it is desirable to be able to directly measure and control the size of the weld pool. Because precise control of the weld pool size produces a correspondingly precise control of weld penetration and thus, the quality of the weld significant effort has been expended to measure directly the weld pool characteristics. However, weld pool characteristics are determined by a wide variety of parameters such as: size, thickness, shape and metallurgical characteristics of the workpiece; the amount of heat applied to the workpiece by the welding torch or arc; the impurities in the workpiece, the weld rod and gas cover; the rate of movement of the welding apparatus and/or workpiece; etc. In the past, a skilled welder watched the weld pool and made appropriate adjustments based on experience. However, with the advent of automatic welders and more exacting welding tolerances it has become necessary to instrument what once was done visually.
It will be appreciated by those skilled in the art that one way to produce high quality welds is to monitor the characteristics of the weld pool. If the weld pool exceeds certain predetermined characteristics this deviation can be sensed by appropriate instruments that can feed the information back to automatic equipment to apply compensating adjustments in the welding parameters to maintain weld quality.
A major difficulty encountered in viewing weld pools arises from the severe gradients of light intensity that are developed in the presence of the welding arc. The brightness of the arc generally either overpowers the average brightness of the weld pool or overloads the sensing device. Various sensing devices coupled with wide range of geometries and positioning of the sensing devices have been described to suppress entry of the entire arc light into the sensing device and to extract information about weld pool characteristics. These include the use of various optical elements such as neutral density filters, viewing the weld pool at an oblique angle relative to the welding torch or through the torch, viewing the weld pool with infrared sensors or at selected infrared wavelengths, using spot thermal sensors, and various combinations and permutations thereof as set forth in U.S. Pat. Nos. 5,275,327, 3,627,972, 4,477,712, 5,475,198, 4,975,558, 4,831,233, 5,481,085, 4,737,614, 4,484,059, 4,611,111, 4,767,911, by way of example. In practice the geometry of the surface of the molten weld pool is constantly changing and moreover, the weld pool itself has a specular surface which makes reliable extraction of the weld pool boundary from reflection measurements difficult.
One of the more successful approaches to measuring weld penetration has been to place an infrared sensor at the backside of the workpiece, i.e., on the opposite side of the workpiece from the torch. In this way problems associated with entry of the entire arc light into the sensing device are avoided. As the weld pool penetrates the thickness of the workpiece, its presence can be sensed by radiation detectors such as infrared sensors and appropriate adjustments in welding parameters can be made. However, this approach requires accessibility to the backside of the joint and adequate accessibility is not always available on the hardware to be welded, for example, where two halves of a hemisphere must be welded together. The use of infrared sensors to detect penetration of the weld pool presents additional problems.
As the weld pool begins to penetrate the workpiece the temperature at the backside of the joint increases and infrared radiation begins to be emitted from the backside of the workpiece. The amount of infrared radiation as well as the area radiating increases as the weld pool approaches the backside. The problem now arises of differentiating the weld pool boundary from the general background of infrared radiation being emitted in order to determine the extent of penetration of the weld pool.
What is needed is a method for quantitatively measuring the size of the weld pool at the backside of a workpiece being welded in order to control weld penetration and thus, the quality of the weld. The method should further provide for easy access to the backside of a workpiece even for a workpiece having a complex geometry, such as two halves of a hemisphere.